The need and use of carbohydrates in many biochemical pathways has been extensively studied and reported over centuries. Mono-, di-, and polysaccharides, or sugars, are a prime dietary source of carbohydrate for many organisms.
Glucose is one of the more readily available sugars and its structure lends itself to be readily acted on by the biochemical systems of many organisms. Comprised of six carbon atoms, glucose falls within the category of aldehexoses. Aldehexose has four chiral centers which lead to 16 stereoisomers. Two stereoisomers of aldehexoses are regarded as glucoses, the major one being D-glucose. All major dietary carbohydrates contain glucose, either as their only building block, as in starch and glycogen, or together with another monosaccharide, as in sucrose and lactose. The metabolism of this carbohydrate translates into energy, such as adenosine triphosphate (ATP). Other metabolic routes for glucose lead to energy storage. The glucose molecule can exist as an open-chain (acyclic) form or in a ring (cyclic) form.
Glucose may be used as a precursor for the synthesis of several important substances, such as starch, cellulose, and glycogen. Lactose, a sugar in milk, is a glucose-galactose disaccharide. Sucrose, another disaccharide, joins glucose to fructose. While glucose is the major transport form of sugars in metazoa, sucrose and its derivatives serves as the major transport form in plants.
Glucose is one of the downstream products of photosynthesis in plants and some prokaryotes. In eukaryotes, such as animals and fungi, glucose may be produced as the result of the breakdown of glycogen, through a process referred to as glycogenolysis. In plants, the resulting breakdown substrate is starch. Glucose may also be derived from the action of invertase on the major transport sugar sucrose in plants (in the cell wall, the cytosol or vacuole, each by a specific isoform).
In animals, glucose may be synthesized in the liver and kidneys from non-carbohydrate intermediates, such as pyruvate and glycerol through a process referred to as gluconeogenesis. Glucose may also be synthesized, such as through enzymatic hydrolysis of starch. Commercially, crops such as maize, rice, wheat, potato, cassava, sago, and arrowroot may be used as a source of starch.
Glucose may be used in either aerobic or anaerobic respiration. Carbohydrates are a significant source of energy for organisms. Aerobic respiration can provide roughly 3.75 kcal of energy per gram. Breakdown of carbohydrates, such as starch, results in monosaccharides and disaccharides. Through the process of glycolysis and the reactions of the citric acid cycle (or Krebs cycle), glucose is oxidized and broken down to eventually forms carbon dioxide and water, yielding energy sources, predominantly ATP. The insulin reaction, as well as other mechanisms, may regulate the concentration of glucose in the blood.
The need for energy in neurological centers, such as the brain, directly correlates glucose to psychological processes. Glucose is a primary source of energy for the brain, and hence its availability influences psychological processes. When glucose is low, psychological processes requiring mental effort may be impaired. Both aerobic and anaerobic respiration start with the early steps of the glycolysis metabolic pathway, the first step being the phosphorylation of glucose by hexokinase to prepare it for later breakdown to provide energy. The immediate phosphorylation of glucose by a hexokinase may then prevent diffusion out of the cell. The act of phosphorylation adds a charged phosphate group, thereby preventing the glucose-6-phosphate from easily crossing the cell membrane. Glucose is also important for the production of proteins and in the process of lipid metabolism. Glucose may also serve as a precursor molecule for ascorbic acid, or vitamin C.
Accordingly, the uptake, absorption, processing, metabolism, exchange and transport of sugars, such as glucose and sucrose, within a cell and between cells of a tissue in an organism is of utmost importance for the ability of a cell or the organism comprising the cell to thrive. Dysfunction of glucose or sucrose transport across cell membranes and between the organelles of a cell can be catastrophic. There is a need to develop methods to regulate the transport of glucose efficiently.